I'm not very knowledgeable in vintage prototypes, but does this auction seem legit? Is this something that Kenner would normally do? It says it comes from a former Kenner employee, but what kind of proof could the seller provide about that?

It's certainly not beyond the realms of possibility that it is genuine as there are a number of examples of Kenner using different cardbacks for "testing" how the figure will look on display(ie bubble size and weight of figure/punch hole location, on the card etc).One example is a Han Trenchcoat figure on an Emperors Royal Guard cardback.Obviously i'd want to see the card for evidence of a reseal.Not certain if all figures tested in this way were ones with no copyright or whether some were regular figures.